Bilingual site with JS

I needed to put a bilingual website and I found this script below, which by the way helped a lot, but it always shows the two versions "BR" "ENG" before "hiding" one of them. Even altering the order of events he continues to show both languages. I'm letting something go, but I do not understand. Does anyone have a light?

This is the code:

<script type="text/javascript">
        function createCookie(name, value, days) {
            if (days) {
                var date = new Date();
                date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
                var expires = "; expires=" + date.toGMTString();
            }
            else
                expires = "";
            document.cookie = name + "=" + value + expires + "; path=/";
        }
        function readCookie(name) {
            var nameEQ = name + "=";
            var ca = document.cookie.split(';');
            for (var i = 0; i < ca.length; i++) {
                var c = ca[i];
                while (c.charAt(0) === ' ')
                    c = c.substring(1, c.length);
                if (c.indexOf(nameEQ) === 0)
                    return c.substring(nameEQ.length, c.length);
            }
            return null;
        }
        function language(lang_on, lang_off) {
            createCookie("langue_on", lang_on, 365);
            createCookie("langue_off", lang_off, 365);
            for (var i = 0; i < document.getElementsByTagName("div").length; i++) {
                if (document.getElementsByTagName("div")[i].lang === lang_on) {
                    document.getElementsByTagName("div")[i].style.display = "block";
                }
                if (document.getElementsByTagName("div")[i].lang === lang_off) {
                    document.getElementsByTagName("div")[i].style.display = "none";
                }
            }
        }
        function startlanguage() {
            var notdefined;
            var lang_on = readCookie("langue_on");
            var lang_off = readCookie("langue_off");
            if (lang_on === notdefined) {
                lang_on = "pt";
            }
            if (lang_off === notdefined) {
                lang_off = "en";
            }
            language(lang_on, lang_off);
        }
        window.onload = function () {
            startlanguage();
        };
    </script>

I call the function like this:

<div lang="en">ENGLISH!</div>
<div lang="pt">PORTUGA!</div>